Ignoring for the moment the recent explosive splitting of the genus Scilla, I think Jan (who lives across the city from me) has the same thing that's flowering in my rock garden now. I'm pretty sure, given the botanical description and the history of my bulb collection, that it's Scilla lilio-hyacinthus, with S. verna another possibility. I'll find out more when it goes dormant and can be lifted, as S. lilio-hyacinthus is described as having distinctive scale-like structures on the bulb. Either one could have got into my rock garden as random seedlings when I moved to the present place.